In the design of integrated circuits having MOS (metal-oxide semiconductor) transistors, it is sometimes important to provide a stable bias voltage to a substrate of the integrated circuit. One reason to provide a bias voltage is to prevent local coupling that may inadvertantly forward bias PN junctions on the integrated circuit. Another reason to provide a bias voltage to the substrate of an integrated circuit is to control the threshold voltage (V.sub.T) of the MOS transistors. The V.sub.T of a MOS transistor is the minimum gate voltage required to form a conductive channel between the source and drain regions. The V.sub.T of a MOS transistor may be varied to improve performance of an integrated circuit. As integrated circuits having MOS transistors are required to operate at lower power supply voltages it becomes more important to be able to control V.sub.T accurately. Also, as the size of the MOS transistors are reduced (below about 0.5 micron), in an effort to increase density, the V.sub.T becomes very sensitive to changes in the substrate bias voltage.
A typical substrate bias circuit includes a level detection circuit, an oscillator, and a charge pump. The level detection circuit monitors the level of the substrate bias voltage and provides a control signal to activate or deactivate the oscillator. When activated, the oscillator provides timing signals to control the output of the charge pump. The output of the charge pump is fed back to the level detector to control the level of the substrate bias voltage.
However, the typical level detection circuit does not provide the accuracy needed in order to precisely control and stabilize V.sub.T when the MOS transistors are scaled down. Also, temperature and process variations can change the operating characteristics of integrated circuits, and can produce wide variations in MOS transistor performance. In addition, variations in the power supply voltage can affect the output of the substrate bias circuit, making it more difficult to provide a stable substrate bias voltage.